In later years, Wilberforce supported the campaign for the complete abolition of slavery, and continued his involvement after 1826, when he resigned from Parliament because of his failing health. That campaign led to the Slavery Abolition Act 1833, which abolished slavery in most of the British Empire; Wilberforce died just three days after hearing that the passage of the Act through Parliament was assured. He was buried in Westminster Abbey, close to his friend William Pitt.

In June 1786 Thomas Clarkson published Essay on the Slavery and Commerce of the Human Species, Particularly the African. As Ellen Gibson Wilson has pointed out: “A substantial book (256 pages), it traced the history of slavery to its decline in Europe and arrival in Africa, made a powerful indictment of the slave system as it operated in the West Indian colonies and attacked the slave trade supporting it. In reading it, one is struck by its raw emotion as much as by its strong reasoning.” William Smith argued that the book was a turning-point for the slave trade abolition movement and made the case “unanswerably, and I should have thought, irresistibly”.

The House of Commons agreed to establish a committee to look into the slave trade. Wilberforce said he did not intend to introduce new testimony as the case against the trade was already in the public record. Ellen Gibson Wilson, a leading historian on the slave trade has argued: “Everyone thought the hearing would be brief, perhaps one sitting. Instead, the slaving interests prolonged it so skilfully that when the House adjourned on 23 June, their witnesses were still testifying.”

James Ramsay, the veteran campaigner against the slave trade, was now extremely ill. He wrote to Thomas Clarkson on 10th July 1789: “Whether the bill goes through the House or not, the discussion attending it will have a most beneficial effect. The whole of this business I think now to be in such a train as to enable me to bid farewell to the present scene with the satisfaction of not having lived in vain.” Ten days later Ramsay died from a gastric haemorrhage. The vote on the slave trade was postponed to 1790.

Wilberforce believed that the support for the French Revolution by the leading members of the Society for the Abolition of Slave Trade was creating difficulties for his attempts to bring an end to the slave trade in theHouse of Commons. He told Thomas Clarkson: “I wanted much to see you to tell you to keep clear from the subject of the French Revolution and I hope you will.” Isaac Milner, after a long talk with Clarkson, commented to Wilberforce: “I wish him better health, and better notions in politics; no government can stand on such principles as he maintains. I am very sorry for it, because I see plainly advantage is taken of such cases as his, in order to represent the friends of Abolition as levellers.”

In March 1796, Wilberforce’s proposal to abolish the slave trade was defeated in the House of Commons by only four votes. At least a dozen abolitionist MPs were out of town or at the new comic opera in London. Wilberforce wrote in his diary: “Enough at the Opera to have carried it. I am permanently hurt about the Slave Trade.” Thomas Clarkson commented: “To have all our endeavours blasted by the vote of a single night is both vexatious and discouraging.” It was a terrible blow to Clarkson and he decided to take a rest from campaigning.

William Wilberforce introduced an abolition bill on 30th May 1804. It passed all stages in the House of Commons and on 28th June it moved to the House of Lords. The Whig leader in the Lords, Lord Grenville, said as so many “friends of abolition had already gone home” the bill would be defeated and advised Wilberforce to leave the vote to the following year. Wilberforce agreed and later commented “that in the House of Lords a bill from the House of Commons is in a destitute and orphan state, unless it has some peer to adopt and take the conduct of it”.

In 1805 the bill was once again presented to the House of Commons. This time the pro-slave trade MPs were better organised and it was defeated by seven votes. Wilberforce blamed “Great canvassing of our enemies and several of our friends absent through forgetfulness, or accident, or engagements preferred from lukewarmness.” Clarkson now toured the country reactivating local committees against the slave trade in an attempt to drum up the support needed to get the legislation through parliament.

In January 1807 Lord Grenville introduced a bill that would stop the trade to British colonies on grounds of “justice, humanity and sound policy”. Ellen Gibson Wilson has pointed out: “Lord Grenville masterminded the victory which had eluded the abolitionist for so long… He opposed a delaying inquiry but several last-ditch petitions came from West Indian, London and Liverpool shipping and planting spokesmen…. He was determined to succeed and his canvassing of support had been meticulous.” Grenville addressed the Lords for three hours on 4th February and when the vote was taken it was passed by 100 to 34.

Wilberforce commented: “How popular Abolition is, just now! God can turn the hearts of men”. During the debate in the House of Commons the solicitor-general, Samuel Romilly, paid a fulsome tribute to Wilberforce’s unremitting advocacy in Parliament. The trade was abolished by a resounding 283 to 16. According to Clarkson, it was the largest majority recorded on any issue where the House divided. Romilly felt it to be “the most glorious event, and the happiest for mankind, that has ever taken place since human affairs have been recorded.”

Under the terms of the Abolition of the Slave Trade Act (1807) British captains who were caught continuing the trade were fined £100 for every slave found on board. However, this law did not stop the British slave trade. If slave-ships were in danger of being captured by the British navy, captains often reduced the fines they had to pay by ordering the slaves to be thrown into the sea.

Wilberforce made it clear that he considered the African Institution should do what it could to convert Africans to Christianity. In 1811 he wrote: “In truth there is a peculiar call on our sensibility in the present instance, for in proportion as the lot of slaves is hard in the world, we ought to rejoice in every opportunity of bringing them under their present sufferings, and secure for them a rich compensation of reversionary happiness.”

In 1808 the Clapham Set decided to transfer the Sierra Leone Company to the crown, the British government accepted Wilberforce’s suggestion that Thomas Perronet Thompson would be a suitable governor. He introduced an extensive range of reforms and made serious allegations against the colony’s former administrators. Stephen Tomkins, the author of William Wilberforce (2007) has argued: “He (Perronet Thompson) single-handedly abolished apprenticeship and freed the slaves. He filed scandalised reports to the colonial office. Wilberforce told him he was being rash and hasty, and he and his colleagues voted unanimously for his dismissal. Wilberforce advised him to go quietly for the sake of his career.”

At the conference in May 1830, the Society for the Mitigation and Gradual Abolition of Slavery agreed to drop the words “gradual abolition” from its title. It also agreed to support the plan put forward by Sarah Wedgwoodfor a new campaign to bring about immediate abolition. Wilberforce, who had always been reluctant to campaign against slavery, agreed to promote the organisation. Thomas Clarkson praised Wilberforce for taking this brave move. He replied: “I cannot but look back to those happy days when we began our labours together; or rather when we worked together – for he began before me – and we made the first step towards that great object, the completion of which is the purpose of our assembling this day.”

William Wilberforce died on 29th July, 1833. One month later, Parliament passed the Slavery Abolition Actthat gave all slaves in the British Empire their freedom. When Thomas Clarkson heard the news he locked the door of his study and his wife heard him “in an agony of grief weeping and uttering loud lamentations.”

In 1834 Robert Wilberforce and Samuel Wilberforce, began work on their father’s biography. The book was published in 1838. As Ellen Gibson Wilson, the author of Thomas Clarkson (1989), pointed out: “The five volumes which the Wilberforces published in 1838 vindicated Clarkson’s worst fears that he would be forced to reply. How far the memoir was Christian, I must leave to others to decide. That it was unfair to Clarkson is not disputed. Where possible, the authors ignored Clarkson; where they could not they disparaged him. In the whole rambling work, using the thousands of documents available to them, they found no space for anything illustrating the mutual affection and regard between the two great men, or between Wilberforce and Clarkson’s brother.”